Plastering Resource Library

The Stucco Solution

Throughout history, those in the building trades have built structures that would last a lifetime. This is apparent from a large number of buildings and houses that are well over 50 years old and remain occupied. In today’s building industry, it is essential that members of all the various trades strive for this kind of quality now more than ever. Although model building codes may establish minimum standards for building, it is the responsibility of both the industry and the individual contractors to create standards that exceed this minimum. Because of the competitive climate, many contractors are willing to sacrifice quality and pride in their trade for increased production and profits. For the last sixty years, we have known the correct procedure for constructing a stucco wall. We know that superior results are achieved through the use of quality materials and techniques in the hands of trained, skilled tradesmen. Inflation and competition have forced contractors to look for ways to reduce costs, and many have chosen to use inferior materials and untrained labor. Unfortunately, the ensuing result is usually an unsatisfactory product, which translates into problems later.

The Making Of Quality Stucco

In 1937, Byron Dalton wrote and published a book entitled, Practical Plastering and Cement Finishing. In his book, he not only explains the characteristics and the importance of good, quality stucco, but he also supports the fact that the nature of stucco, and the appropriate methods of working with it, have not changed significantly over the years. He presents his views in the following statement:

“Properly applied, portland cement stucco provides a protective shell of concrete, giving the structure desirable qualities inherent in that material. Fire safety, durability, water tightness, and economy are secured by means of this protective wall covering.”

Dalton further states:

“Durability and water tightness of portland cement stucco are dependent upon two primary factors in addition to an adequate base. First, the proper application of flashing and use of projecting trim; second, adherence to the specification for materials, mixing, application, and curing of the stucco. Convincing proof that Portland cement stucco, when applied in accordance with recommended practices, furthers those desirable qualities for which it is so well known, as evidenced by its extensive use in modernization and new consideration.”

Although there is much evidence to support Dalton’s regard for the performance of stucco wall assemblies, I have often wondered why houses built before 1980 seems to be outperforming those that are built today, in terms of cracks and porosity. My curiosity brought me together with Steve Roland of Arnel Homes, and the two of us decided to do research on stucco and learn more about how it was applied prior to 1980.

In time, we constructed 40 panels of stucco using a variety of materials and methods of application. Our aim was to determine which combinations of materials, and which methods of application, would perform the best as well as be cost-effective. The test results indicated that the only way to achieve high-performance results with stucco was by using clean, well-graded sand in the correct proportions and employing only skilled labor to apply the mixture.

Test Procedures

The 40 test panels constructed were arranged in 10 groups of four. The majority of the panels measured 32-inches by 28-inches, and were trimmed out with Milcor and lathed with paper and netting nailed with furring nails; four of the panels were full height panels measuring 48-inches by 96-inches. On panel groups one through nine, the materials were applied by hand; on panel group 10, a plaster gun or pump was used for the application.

Omega Stucco Company, of Santa Ana, Calif., supplied plastic cement (UBC standard 25.1) and common cement (ASTM C-150 type II portland cement) for the test mixes. Kelcrete admixture furnished by Kel Crete Industries and lime was also used for a portion of the stucco mixes. Tim Folks, technical services manager of Riverside Cement Co., coordinated testing of the panels at both the test site and at Riverside’s lab. Four types of sand were used and tested for gradation and sand equivalency. The four varieties of sand included SEs of 48, 57, 77 and 86. The volume of sand put into the mixer was verified with a 1 cubic foot test container. The total volume of water added was equal to the sum of the measured moisture in the sand pile and the additional water required to achieve the correct slump. A three-minute mixing time was verified with a stopwatch beginning after half of the sand and admixture had been added. While the plaster was fresh, several different properties were measured for evaluation. The slump was measured to determine the relative workability of the plaster mix. Also, plaster temperatures were measured and recorded.

Through the course of the testing, a number of additional properties were also measured. Air content, which affects the pumpability, workability, and strength of the plaster, is among the many factors that were measured. Test cylinders were made of each mixture in order to determine the compressive strength potential. Also, a full cement content analysis was run on each panel to determine the unit weight, percent by weight of cement, percent by weight of sand, and percent by weight of water, in addition to the sand-to-cement ratios, water-to-cement ratios, and the volume percent of permeable voids.

Furthermore, a water test was used to determine the relative water absorption, the path of water flow in the plaster panels, and depth of water penetration of the plaster panels. For the water test, two 18-inch tall, 4-inch diameter ABS pipes filled with Phenolphthalein-dyed water adhered to the surface of each panel. The panels were later cross-sectioned to reveal the degree and path of water penetration through the stucco.

Importance Of Clean Sand

The most telling result of the testing was the vital role that sand quality plays in achieving a quality stucco mix. Sand composes 80 percent of a stucco wall, and because it is the primary ingredient, it is vital that the quality of the sand be controlled.

Imagine a bucket filled with tennis balls and different sized marbles. The marbles are filling in the spaces between the tennis balls. Cement paste, a desirable fine, is filling in the remaining spaces, while at the same time bonding to the tennis balls and the marbles. In a similar fashion, cement paste will hold together various sized aggregates of sand better than it will uniformly fine sands. The objective is to be rid of the spaces, or voids, and to create a denser, stronger wall. Cement paste is preferable to fine sand when it comes to filling in these voids because cement is the glue that holds the aggregate, or the sand, together.

Dalton tells us that “sand used in stucco should not have an excessive amount of fines or silt”. Fines or silt are objectionable and are undesirable, as they prevent the cement paste from binding together the particles of sand, thus reducing the strength of the cement mortar and making it more porous. In cement plaster, the weakest point in the compound is where the cement paste attaches to the aggregate; fine particles of sand act to coat the larger particles of aggregate, thereby preventing the cement paste from sticking to the aggregate.

Fine sands may appear to contribute to high compressive strength, but this is deceptive because, at the same time, they are contributing to weakening the flexural strength. Sand with excessive fines is also more likely to cause plaster to develop shrinkage cracks because the finer particles require more water to be added to achieve a workable mix.

The reason for this is that the finer particles have a greater amount of surface area than the larger particles. The greater the surface area, the more water that has to be added to the mix. However, when there is too much water in a mix, the excess liquid will evaporate as the stucco base coat dries, thus leaving voids behind. And some fines, such as fine clay, will actually soak up water, thus causing them to expand when wet and shrink when dry, adding to the porosity of the cement plaster.

The relationship between fine sands and the demand for water were clearly illustrated in the testing. As a rule of thumb,” one sack of cement requires a minimum of 2.6 gallons of water, or 23% of water by weight, in order for the cement to hydrate. Typically, plasterers add between 5.25 to 10 gallons of water to a batch of stucco. Any water in excess of the required 2.6 gallons serves only to increase workability. Most of our test batches using sand with an SE of 87 needed 5.5 gallons of water to achieve a 2V2-inch slump. Those test batches using sand with an SE of 49 needed 7.5 gallons of water to achieve a 2V2-inch slump. Batch lA, consisting of 1 sack of common cement and 10 percent lime, and using sand with an SE of 49, required 10.25 gallons of water to achieve a 2Yz-inch slump. Clearly, mixes made with a dirty aggregate demanded more water to achieve a similar slump.

 

Observations Of A Stucco Wall

The four photographs below and on page 35 demonstrate how cleaner sand and less water contribute to fewer shrinkage cracks in the scratch coat. The panels show progressive improvement from 2A to 2D, with 2D showing the least amount of cracks. All panels were made with the same mix of common cement sand; the only differences between the panels were the sand quality and the amount of water added to achieve the desired workability.

Panel 2A was made with 49 SE sand and required 7.25 gallons of water to achieve a slump of 2% inches. Not only does it have a large number of shrinkage cracks, but leaves a great deal of wire exposed. Panel 2B was made with 57 SE sand and 7.25 gallons of water to achieve a slump of 3 inches. Although it shows fewer cracks, there are still too many.

Panel 2C was made with 77 SE sand and required 7 gallons of water to achieve a 31 1/4-inch slump.

Panel 2D, with the least amount of shrinkage cracks, was made with 87 SE sand and required only 5.5 gallons of water to achieve a 2%-inch slump. Of note, even though panels 2A and 2B had the same quantity of water added, the plasters used on the two panels had different slumps because of the higher SE in panel 2B.

The results obtained from the scratch coat panels are particularly revealing in that the mud is laid on, scored, and left alone. Conversely, the brown coat may be rodded, floated or possibly worked in order to close up the shrinkage cracks. Working, or densifying, the scratch and brown coats by floating after installation will contribute to a denser stucco base coat wall. Floating acts to recompact the brown coat as the excess water evaporates from the wall. Moist cure is also needed to achieve a hard, rigid cement. Moist curing improves tensile strength, as well as density because the cement matrix is allowed to fully hydrate. The photos below and at right illustrate how proper application techniques can improve the overall quality of cement plaster. Panel 6B is rodded, but not floated or moist cured. The panel looks rougher, is visibly porous, and has excessive shrinkage cracks. Panel 3D was thoroughly floated and clearly shows fewer shrinkage cracks. It should be noted that panel 6B used SE 49 sand, while panel SE was made with SE 87 sand.

The Value Of Dense Stucco

One evening, while moist curing the panels, Steve Roland and I noticed that there was light reflecting off of certain panels, but not others. Upon closer examination, we saw that the panels that were floated had puddles of water on top of them. Conversely, panels that had only been rodded had no water puddling on top. We observed that the panels that had been floated did not absorb the water, while the panels that were only rodded and scraped soaked up water immediately. When the panels were cross-sectioned following the water penetration testing utilizing the Phenolphthalein-dyed water, the pink trails through the stucco panels supported this observation. The pictures on page 38 illustrate the pronounced difference in water penetration between dense stucco (panel 3D) and more porous stucco (panel 8A). Panel 3D, made with half plastic and half common cement and SE 87 sand, shows a good, tight mix with minimal water migration. Panel 8A, made with plastic cement and SE 49 and shows pronounced water penetration. Further, the pink dye is visible at the wire, but not above the wire, thus indicating that the water uses the lath as a pathway to disperse itself. That pathway is aided by voids around the lath wire that were created by the mix shrinking away from the lath. As excess water evaporates out of the mix, the mix simultaneously shrinks away from dissimilar materials. Another cross-section photograph of panel 9A (on page 38) shows a clear line and possible separation between the scratch and brown coats.

It also shows large voids around the wire. As evidenced by this cross-section, the brown coat should be applied at the proper time. This occurs within 24 hours after the scratch coat’s initial shrinkage cracks have formed; and only when the scratch coat is hard and rigid enough to take the brown coat. One must also be sure to moist cure as needed for hardening. A better bond will occur between the scratch and brown coats if the scratch coat is lightly dampened before the application of the brown coat. When the scratch coat is troweled, it will be a richer mix and will be more likely to seal properly, although the score lines do provide more surface area to which the brown coat must bond. It is difficult to get this just right because, although you want a rich mix, the troweling process causes fines and lime to come to the surface and produced a glazed appearance. The wisest solution is to score after some of the excess water has evaporated from the wall but before the scratch coat tears.

Conclusions

Stucco is still the least expensive and most practical wall covering in existence today. It is extremely durable and is available in a wide variety of colors and textures. Stucco is also very forgiving, even if parts of the stucco wall system are missing or installed incorrectly. If the structure and the substrate are of sound construction, the stucco wall system will perform as intended. The materials used in preparing stucco, sand, and cement are easily accessed almost anywhere in the world. While stucco is a monolithic wall covering, it is also easy to patch or repair.

In order to achieve a first-rate, dense stucco, certain guidelines need to be followed. Using clean, washed plaster sand that meets ASTM specification C-897 will contribute to a denser stucco base coat due simply to the fact that sand with fewer fines has less water in the mix. When there is less water to evaporate out of the mix, fewer voids will form. It has been demonstrated that floating the scratch and brown coats will also contribute to a denser base coat wall. A moist cure is also needed to achieve a hard, rigid cement. Water will penetrate stucco through cracks, voids and air spaces in the stucco walls. The only way to avoid water penetration is to reduce the number of cracks. There are two, and only two reasons that stucco cracks. First of all, there are cracks caused by normal shrinkage of the mortar as the water evaporates from the wall.

Secondly, there are physical forces applying pressure to the stucco itself. These pressures may come from building movement resulting from plywood shear warping, green or wet lumber shrinkage, excessive loads on outside walls, or walls that are not tied in. The plasterer can assemble the best stucco wall system possible, but if the building structure is of poor quality, or the building is sitting on ground that is moving or sinking, the stucco wall will not perform to its fullest capability. A few plasterers like to use sands with low SEs, often as low as 33. They can mix sand to cement ratios as high as 9: 1, and are able to get the mortar to flow through the pump. This allows them to trowel, spread and float it more easily. We all know that sand is less expensive than cement and that using high ratios of sand offers a quick way to reduce costs. While this provides a short-term method of increasing profits, over the long term the use of dirty sand with low SEs is bound to create problems.

Poor quality sand in the mix can be overcome by floating to densify the mix and moist curing to harden and fill in the voids in the stucco base coat.
Though one might ask, “Is it worth the extra labor?” or “Can I really rely on the plasterers to float the brown coat?”, it makes straightforward sense to use clean sand in correct ratios in the first place. Ultimately, plastering contractors will save themselves work and headaches, while building a fine reputation in the business.